1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus with an oxygen pump type air-fuel ratio sensor formed by an ion conductive solid electrolyte to detect an air-fuel ratio by measuring an oxygen concentration in exhaust gas from an internal combustion engine.
2. Discussion of Background
It has been known to control the engine for, for instance, an automobile at a theoretical air-fuel ratio with use of an oxygen sensor formed by an ion conductive solid electrolyte such as a stabilized zirconia and by detecting a state of burning at a theoretical air-fuel ratio on the basis of the change of an electromotive force produced by the difference between a particle pressure of oxygen in exhaust gas and a partial pressure of oxygen in air. However, although a large output is obtainable when the air-fuel ratio A/F as a proportion in weight of air and fuel is at a theoretical air-fuel ratio of 14.7, there is substantially no change of output in the other area of air-fuel ratio. Therefore, it is impossible to utilize the output of the oxygen sensor when the engine is operated at the air-fuel ratio other than the theoretical air-fuel ratio.